The present invention relates to a lightning arrester device for power transmission line positioned on a steel tower to protect the power transmission line; more particularly, it relates to a lightning arrester device which performs the disconnection of a lightning arrester from the power transmission line at the time of the fault of the lightning arrester.
In general, an aerial ground wire is positioned on the power transmission line to protect it from the attacking of direct lightning. However, when the lightning current is large, the electric potential of the steel tower which is normally in ground potential increases. Therefore, a so called reverse flashover is caused by the elevation of the electric potential over the voltage of the power transmission line whereby the earthing condition results in the system to pass the earth current. Therefore, it has been proposed to cut-off the earth current with a circuit interrupter connected to the power transmission line and then re-close the circuit interrupter.
In a new power transmission line for high voltage and large capacity power transmission, the critical capacity for power transmission depends upon a transient stability of the system at the reclosing time of the circuit interrupter.
In order to improve the transient stability, it is necessary to prevent the trouble of the reverse flashover. One attempt was to connect the lightning arrester device to the power transmission line. As is well known, the conventional lightning arrester device has a structure connecting a serial gap and a functional element made of silicon carbide (SiC) in series. The floating capacity of the serial gap is usually small as 10 PF and accordingly, the discharge characteristic of the gap is easily changed depending upon the condition of the surface such as a dust and a broken surface condition of the insulator which holds a lightning arrester element. Thus, it is necessary to work periodically for a maintenance. When a functional element made of silicon carbide is used, several hundreds Amp. of a dynamic current is passed under the normal voltage to ground, and accordingly, a perfect earth current cancellation can not be disadvantageously attained. Therefore, this conventional lightning arrester device has not been practically applied in the power transmission line.
Recently, a sintered product made of a main component of zinc oxide (ZnO) and a minor component such as bismuth, antimony, cobalt, etc. (hereinafter referring to as ZnO element) has been developed. The ZnO element has an excellent non-linearity of voltage-current characteristic and a lightning arrester element can be prepared by using the ZnO element so as to decrease a leakage current passed in the insulator under the normal voltage to ground to several tens .mu.Amp. Accordingly, it is no longer necessary to form the serial gap required in the conventional lightning arrester. The disadvantage found in the application of the conventional lightning arrester device to the power transmission line can be overcome by using a zinc oxide type lightning arrester device. That is, the dynamic current of several hundreds Amp. as found in the conventional device is not passed under the normal voltage to ground and it can be considered as a non-dynamic current type lightning arrester device. Accordingly, no disturbance results in the power transmission line system because the lightning arrester device responds to only the pulse of the lightning current.
Furthermore, the lightning arrester device has not the serial gap found in the conventional device whereby the lightning arrester device has a stable performance without being affected by external conditions.
However, even though it is the ideal lightning arrester device as above-mentioned, it absorbs an abnormal voltage caused by the lightning. The lightning arrester should be sometimes able to treat a current higher than the estimated lightning current even though the possibility of the occurrence is low. In such case, the ZnO element may be broken. When the ZnO element is broken, the ON stage results between the terminals of the device and the earth current is passed under the normal voltage to ground. It is necessary to disconnect immediately the lightning arrester device from the power transmission line system when this abnormal condition is caused.